1. Field of the Invention
The present invention relates to a method of directly or indirectly applying a liquid or pasty medium to a continuous material web.
2. Related Art
Methods according to the class described above are, in practice, performed as part of so-called coating plants in order to provide one or both sides of a continuous material web, composed for example of paper, cardboard or a fabric material, with one or more layers of the medium, for example ink, starch, impregnating fluid or the like.
So-called direct application involves an applicator applying the liquid or pasty medium directly onto the surface of the continuous material web supported during application on a revolving counter-surface, such as an endless belt or a counter-roll. Indirect application of the medium, on the other hand, first involves applying the liquid or pasty medium onto a carrier surface, e.g. the surface of a counter-roll designed as an application roll, from where, in a roll gap through which the material web passes, it is transferred by the application roll to the material web.
The "Paper Maker's Pocket Book", March 1993 edition, pp. 206 and 207, Section 5.7.2.3.5, describes a method according to the class for directly or indirectly applying a liquid or pasty medium to a continuous material web; in this method, a desired transverse and/or longitudinal profile of the medium to be applied or which has been applied is set by means of an application unit (here: a so-called smoothing scraper type coating device) which comprises an applicator, a doctor element, e.g. a doctor knife or a scraper blade, downstream of the applicator, and a doctor element support fixed to a pivotable support or doctor beam and co-pivotable therewith. The doctor element is held on this support. For this purpose, the doctor element is moved into a predetermined adjustment position by pivoting the entire support beam and the doctor element support attached thereto and pressed at a predetermined contact pressure against a continuous counter-surface, e.g. the surface of an application roll or the surface of the material web itself. The applicators used in conjunction with such a process are also usually equipped with one or more actuators distributed over the applicator's longitudinal extension, e.g. mechanical, electrical, electromagnetic, hydraulic or pneumatic actuators and the like, which are attached to the support beam and act upon the doctor element evenly across substantially the entire material web width or zonally to varying extents for the precision adjustment of the contact pressure exerted on the doctor element, thereby enabling the (precision) adjustment of the transverse and/or longitudinal profile, regardless of the setting of the support beam itself. Not only due to the ever present manufacturing inaccuracies of the applicator and the material web to be coated, not to mention doctor element wear and the associated local variations in doctor element contact pressure against the coated counter-surface, but also due to changing operational and production conditions, it is nevertheless necessary to correct the transverse and/or longitudinal profile continuously. This is done in the known method by altering the originally chosen support beam adjustment position and/or as a result of evenly or unevenly altering the forces exerted upon the doctor element by the actuators. In other words, the support beam adjustment position or the actuating positions of the actuators which act upon the doctor element are permanently re-adjusted during the application unit's continuous operation. It is evident that this method not only requires considerable outlay in terms of control technology and structural design, entailing a number of complex and expensive actuators, and hence is quite cost-intensive, but also leads to problems in terms of reproducibility of the settings selected for specific operating conditions and parameters.
It should be noted that the aforementioned method is in principle applied both for the "stiff blade approach" and for the "bent blade approach". The stiff blade technique involves a doctor element, which usually has an inclined edge on its contact surface, being pressed at a specific working or doctor angle against the counter-surface to be doctored and this angle is also usually retained when a change in contact pressure is needed to correct the coating weight or the transverse and/or longitudinal profile. The bent blade technique, on the other hand, involves pressing an elastically deformable doctor element, normally a doctor knife, at a smaller angle against the counter-surface to be doctored and varying the contact pressure and/or the working or knife angle, depending on the coating weight required. In the stiff blade technique, the knife angle is usually approx. 25-45.degree. and in the bent blade technique approx. up to 25.degree.. The settings mentioned should, however, only be understood as rough reference values.
The problems described above in conjunction with the reproducibility of the adjustments to be selected for specific operating conditions and parameters are particularly serious in the aforementioned bent blade approach with regard to the coating quality to be achieved.
DE 29 13 421 B2, DE 30 362 74 C2 (which corresponds to the subject matter of this document) and WO 94/03282 disclose a method of applying a liquid or pasty medium to a continuous material web; this method substantially corresponds to the well-known process described at the outset, but instead of the entire support or doctor beam, just a correspondingly adjustable doctor knife support is pivoted.
A method according to the class is also known from EP 0 617 168 A1. In this method, an entire metering device of the applicator, at which there is fixed a doctor element to be operated in the bent blade mode, is analogously pivotable and adjustable instead of the support or doctor beam or a correspondingly pivotable doctor knife support. Separate actuators are in turn provided for the precise adjustment of the longitudinal and/or transverse profile.